This invention relates to a control system for a permanent-magnet motor driven by line-commutated inverter, and more particularly to an adaptive high-pass filter network for use in such a control system to condition back EMF signals utilized to generate inverter control signals.
In some applications, a permanent-magnet (PM) motor is controlled by a line-commutated inverter driven by a dc source comprised of a phase delay rectifier (PDR). The PDR rectifies the 3-phase power from a utility line and delivers the resulting dc current to the inverter over a dc link choke. The dc link choke is designed to provide a smooth current source for the line-commutated inverter. The inverter provides the necessary current to the motor windings.
A motor speed loop is closed around the inverter control, and a change in required speed results in a change in the PDR firing angle. This in turn changes the dc link current, and hence the motor speed. It is possible to vary the current in the dc link from zero to a maximum by adjusting the firing angle of the PDR in response to a gain and loop compensated error signal, where the error is the difference between a speed command signal and a signal proportional to the speed of the motor generated by precision full-wave rectification of the motor back EMF.
The back EMF is also utilized to generate the inverter control signals. Due to commutation overlap in the inverter, the back EMF is distorted. By integrating the back EMF, constant control sinusoidal waveforms devoid of distortion and in phase with the motor line-to-neutral voltage is obtained. However, the integrators provide much more gain to spurious low frequencies than the actual operating frequency. These spurious low-frequency fluctuations are caused by modulation between the PDR ripple and the back EMF frequency. At high motor speeds, these low-frequency fluctuations modulate the motor speed and are further reinforced by the integrators. Thus, in order to operate a line-commutated permanent-magnet motor at high speed, these low-frequency components must be filtered. A single high-pass filter, however, introduces undesirable phase shifts which result in poor motor power factors. The filter break (corner) frequency must be selected such that the phase shift does not cause excessive change in the power factor. It has been found that any single zero location that results in acceptable poer factors will not adequately eliminate low-frequency noise at all speeds. Consequently, there is a need for a high-pass filter whose break frequency can be changed as a function of speed so that adequate low-frequency filtering and system power factors can be obtained at all motor speeds.